Doping-Induced Isotopic Mg¹¹B₂ Bulk Superconductor for Fusion Application

Abstract

Superconducting wires are widely used for fabricating magnetic coils in fusion reactors. Superconducting magnet system represents a key determinant of the thermal efficiency and the construction/operating costs of such a reactor. In consideration of the stability of ¹¹B against fast neutron irradiation and its lower induced radioactivation properties, MgB₂ superconductor with ¹¹B serving as the boron source is an alternative candidate for use in fusion reactors with a severe high neutron flux environment. In the present work, the glycine-doped Mg¹¹B₂ bulk superconductor was synthesized from isotopic ¹¹B powder to enhance the high field properties. The critical current density was enhanced (10³ A·cm⁻² at 20 K and 5 T) over the entire field in contrast with the sample prepared from natural boron.

1. Introduction

Fusion power is one of the most promising energy source candidates to solve global energy problems, considering its safety and green merits compared with conventional mineral energy sources. In the world-class International Thermonuclear Experimental Reactor (ITER) fusion energy project, the superconducting magnet system serves as a key determinant (Figure 1). A high and steady magnetic field needs to be produced to confine the deuterium (D)–tritium (T) burning plasma inside the ITER Tokamak nuclear fusion reactor. According to the previous ITER plan, hundreds of tons of superconducting cables made from NbTi and Nb₃Sn strands have been fabricated to assemble 18 Nb₃Sn toroidal field (TF) coils, a 6-module Nb₃Sn central solenoid (CS) coil, six Nb-Ti poloidal field (PF) coils, and nine pairs of Nb-Ti correction coils (CC) [1,2,3]. ITER is aimed at demonstrating the feasibility of fusion energy, but for the next step, the development of a commercial fusion reactior there is a concern that, after irradiation, ⁹³Nb be transformed into the long-lived nuclide ⁹⁴Nb with a half-life of about 20,000 years [4,5]. Hence, coil maintenance and repairs may become more cumbersome and the recycling of irradiated Nb-based alloys may call for tens of thousands of years of waiting for them to “cool down”. Meanwhile, thicker shielding will be necessary for long-term operation. For the convenience of radioactive waste treatment and environmental protection, the radioactivation properties of superconducting components within the fusion reactor should be taken into account. The superconductivity of MgB₂ was discovered in 2001 [6]. It is well-known for its simple binary chemical composition and much higher critical transition temperature (T_c) of 39 K than that of NbTi at 9.3 K. In order to operate Nb-based low-temperature superconductors, the core of the magnet needs to be cooled down to 4 K. The only eligible cryogen is liquid helium, which is extremely expensive, not always available, and very difficult to handle. In the case of MgB₂, a working temperature as high as 20 K is low enough to achieve acceptable performance. Remarkably, the operating cost is expected to be cut by over 50% by substituting cryocooler-cooled MgB₂ materials at 20 K for liquid-helium-cooled Nb-based superconductors. Therefore, due to the advantages of cost-effectiveness, lower radioactivation, and the shorter decay time of isotopic Mg¹¹B₂, fundamental research on Mg¹¹B₂ superconducting wires will be valuable for improving the efficiency of practical application in high-irradiation environments such as fusion reactors. Nevertheless, the application of the un-doped MgB₂ remains limited by the sensitivity of the critical current density (J_c) to the increasing applied magnetic field [7]. By yielding an enhancement of J_c especially at high field, chemical doping enabled MgB₂ to meet higher demand in practical application, and carbon-containing compounds definitely attracted the most attention within the dopant’s family.

MgB₂ bulks and wires with carbon addition, for instance, malic acid [8], graphene, coronene, or glucose [9,10,11], have been under investigation, motivated by the potential response of carbon atoms (compared to boron) to donate their additional valence electrons to the σ conduction band. Glycine (C₂H₅NO₂, Glycine) was doped into MgB₂ bulks by a series of techniques in our previous study [12,13]. The dominating mechanism for the enhancement of the J_c lied in the MgO formation in advance of the Mg-B solid-solid reaction, and the simultaneously released carbon atoms provided a certain contribution as well, by substitution the B sites in the MgB₂ lattice. Apart from the carbon doping method, a new trial related to the state of the boron precursor has been carried out as well. The MgB₂ wires prepared from laboratory made nano-sized boron achieved the J_c of 10⁵ A·cm⁻² at 5 K and 4 T [14]. Bovone et al. [15] produced boron powder by magnesiothermic reduction of boron oxide in the lab, which proved to be an excellent precursor for MgB₂ wire manufacture independent of the applied technique. Furthermore, commercial carbon-coated amorphous boron powder brought along carbon doping and benefited the J_c of the synthesized MgB₂ bulks with or without Cu doping [16,17].

Considering the type of the original boron powder, isotopic boron has been adopted to determine the effect on the superconductivity and physical properties of MgB₂. The studies of both Bud’ko et al. [18] and Hinks et al. [19] indicated a difference of 1 K in transition temperature (T_c) for the un-doped MgB₂ made of ¹⁰B and ¹¹B. Simonelli et al. [20] investigated the isotope effect on phonon spectra of MgB₂ with Al doping and suggested a difference in Raman shift for the two isotopic forms of MgB₂. Recently, Alarco et al. [21] extended the study to the effect of ¹⁰B, ¹¹B and natural B (mixture of ¹⁰B and ¹¹B) on the phonon frequencies, which exhibited a pronounced isotopic effect for the phonon modes. Compared with conventional Nb-based superconductors, MgB₂ features “low activation” and a much shorter decay time. Within 1 year, the dose rate of MgB₂ materials will be reduced to the hands-on maintenance level, which is desirable for a fusion reactor magnet system [4]. Additionally, because of the reaction ¹⁰B + n → ⁷Li + He (gas) under the heavy irradiation condition, ¹⁰B can no longer guarantee the stability of the MgB₂ superconducting magnet. ¹⁰B isotope is transformed to ⁷Li and He by the neutron irradiation, while ¹¹B isotope is stable against the neutron irradiation without nuclear transformation and can reduce nuclear heating from 2.58 to 0.13 W/cm³ [22].

By replacing ¹⁰B with the isotope ¹¹B, Mg¹¹B₂ superconducting wires will be much more stable in a neutron irradiation environment due to the smaller neutron capture cross-section of ¹¹B [23]. Considering the abundant reserves of ¹¹B on Earth (20 wt % for ¹⁰B, 80 wt % for ¹¹B), the anticipated cost for extracting the isotope from natural boron is expected to be reduced during the chemical synthesis. Mg¹¹B₂ would be a promising candidate material as a lower field poloidal field and correction coil superconducting magnets in a fusion reactor (Figure 1). In view of this, glycine-doped MgB₂ bulks are prepared from natural B (written as ^10.8B) and ¹¹B in this study to improve the critical current density at high field region.

2. Experimental Details

Amorphous ^10.8B (93%–94% purity, 0.6–0.7 μm in size) or ¹¹B powder (amorphous, 99.2% in purity, about 5 μm in size, from Pavezyum Kimya, Istanbul, Turkey), Mg powder (99.5% purity, 100 μm in size), and glycine powder (99% purity) were mixed in the ratio of MgB₂ + 3 wt % Gly. After ground thoroughly in an agate mortar, the mixture was pressed into cylindrical pellets (5 mm diameter and 1.5 mm thickness) under a pressure of 5 MPa. The obtained pellets were then sintered in the differential thermal analysis apparatus (DSC 404C, Netzsch, Boston, MA, USA) at 800 °C for 0.5 h with a heating rate of 10 °C·min⁻¹ and a cooling rate of 40 °C·min⁻¹. The whole process was accomplished under the protection of flowing high-purity Ar gas. The superconducting properties were measured on a superconducting quantum interference device (SQUID–VSM, Quantum Design, San Diego, CA, USA) after the sample was cut into a slab (4 × 2 × 1 mm³). The corresponding J_c values were calculated from the width of magnetization hysteresis loops based on the Bean model J_c = 20ΔM/[a/(1 − a/3b)] [24], where M is the volume magnetization, ΔM is the difference in volume magnetization between the arms of the M–H loop, and a and b are the sample dimensions (a < b).

3. Results and Discussion

Analogous to the un-doped and the Gly-doped Mg^10.8B₂, the Gly-doped Mg¹¹B₂ is composed of MgB₂ as the main phase and MgO as the only impurity phase (Figure 2).

Generally, doping from carbon sources results in the substitution of carbon for boron in the MgB₂ lattice. Substituted carbon atoms normally donate their additional valence electrons (compared to boron) to the σ conduction band, resulting in decreased carrier concentration by filling the holes and decreasing the superconducting gaps. This will reduce the number of holes at the top of the σ bands together with a reduction of the electronic density of states [25], and consequently the transition temperature T_c was supposed to decrease. Previous studies have stated that the MgB₂ samples showed a strong boron isotope effect, as the T_c for Mg¹¹B₂ decreased almost 1 K in contrast with the Mg¹⁰B₂ sample [18,19]. However, the T_c for the Gly-doped Mg¹¹B₂ sample remained at the same level as the doped Mg^10.8B₂ sample shown in Figure 3.

From the viewpoint of the isotope effect, the increase of the phonon frequency is conductive to improve T_c, and the approach of the phonon and coulomb energies will lead to a decrease of T_c. As summarized by Knigavko [26], the T_c would remain stable when the two effects reached a balance in the Gly-doped Mg¹¹B₂ sample. The replaced carbon atoms likely originated from the reaction of Mg and the decomposition product of glycine, 2Mg + CO₂ → C + 2MgO, with the impurity phase MgO generated. Contrary to the common position that the dielectric MgO occupied in most doping systems, i.e., at the grain boundary [27], the MgO particles in the Gly-doped Mg¹¹B₂ sample might be embedded within the MgB₂ grains in the nano-scale dimension. Besides, element mappings for Mg and O on an area with holes are shown in Figure 4a–c. Combined with the distribution of Mg element, the MgO phase was believed to be dispersed homogeneously on the matrix rather than gathered in the hole, which implied good MgB₂ grain connectivity. A thermodynamic calculation has demonstrated that the MgO phase was formed prior to MgB₂, and the study on undoped Mg¹¹B₂ suggested that the ¹¹B accelerated the Mg-B solid-solid reaction below 650 °C [28]. Hence, the MgO particles were mostly included in the growing MgB₂ grains instead of aggregating at the boundary. The size and distribution of MgO allowed them to become effective pinning centers, and as a result, the Gly-doped sample had a significantly improved J_c performance at least twice larger than those of pure MgB₂ over the entire field at 20 K. The measured J_c-H characteristics of the un-doped and the Gly-doped samples at 20 K are illustrated in Figure 4d. A further improvement in J_c was observed in the Gly-doped Mg¹¹B₂ sample, even at the low field. The enhanced J_c should be attributed to the use of high-purity ¹¹B powder as well, in view that Gly-doped sample prepared from high-purity boron shows two times higher J_c than that from low-purity boron powder [29].

4. Conclusions

A glycine-doped Mg¹¹B₂ sample with layered grains was synthesized from isotopic ¹¹B powder. The glycine-doped Mg¹¹B₂ gives comparable critical current density and could be used for fusion reactors because of endurance against neutron irradiation. The results obtained in this work could guide the fabrication of Mg¹¹B₂ wires to be used as magnet coils in fusion reactor systems such as ITER-type tokamak magnets.